[OLK-5.10]mm/page_alloc: count CMA pages per zone and print them in /proc/zoneinfo

Let's count the number of CMA pages per zone and print them in
    /proc/zoneinfo.
    
    Having access to the total number of CMA pages per zone is helpful for
    debugging purposes to know where exactly the CMA pages ended up, and to
    figure out how many pages of a zone might behave differently, even after
    some of these pages might already have been allocated.
    
    As one example, CMA pages part of a kernel zone cannot be used for
    ordinary kernel allocations but instead behave more like ZONE_MOVABLE.
    
    For now, we are only able to get the global nr+free cma pages from
    /proc/meminfo and the free cma pages per zone from /proc/zoneinfo.
    
    Example after this patch when booting a 6 GiB QEMU VM with
    "hugetlb_cma=2G":
      # cat /proc/zoneinfo | grep cma
              cma      0
            nr_free_cma  0
              cma      0
            nr_free_cma  0
              cma      524288
            nr_free_cma  493016
              cma      0
              cma      0
      # cat /proc/meminfo | grep Cma
      CmaTotal:        2097152 kB
      CmaFree:         1972064 kB
    
    Note: We print even without CONFIG_CMA, just like "nr_free_cma"; this way,
          one can be sure when spotting "cma 0", that there are definetly no
          CMA pages located in a zone.

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